The potential of polystyrene-block-polybutadiene-block-polystyrene triblock co-polymer as a base-polymer of polymer inclusion membranes (PIMs)

https://doi.org/10.1016/j.seppur.2019.115800Get rights and content

Highlights

  • A new base-polymer was used to prepare polymer inclusion membranes (PIMs).

  • The base-polymer was polystyrene-block-polybutadiene-block-polystyrene (SBS).

  • The compatibility between SBS and different common extractants was assessed.

  • SBS-based PIMs showed superior performance than PIMs containing other base-polymers.

  • The thermal properties and surface morphology of SBS-based PIMs were characterized.

Abstract

The use of polystyrene-block-polybutadiene-block-polystyrene triblock co-polymer (SBS) as the base-polymer of polymer inclusion membranes (PIMs) is studied for the first time. SBS is an inexpensive and accessible thermoplastic elastomer which possesses the required properties to be used as a base-polymer in PIMs, namely mechanical strength and high resistance to acids/bases. Its compatibility with commonly used extractants in PIMs was assessed and it was observed that successful PIMs could be obtained when di(2-ethylhexyl)phosphoric acid (D2EHPA), LIX84I or tri-n-octylamine (TOA) were used as the extractants. The performance of these PIMs was investigated for the extraction of Zn(II), Cu(II) and Cr(VI), respectively, although only the TOA-based PIMs did not perform well. Both D2EHPA- and LIX84I-based PIMs were characterized by water contact angle measurements, atomic force microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. In comparison with PIMs containing other base-polymers, the SBS-based PIMs were found to extract faster and more Zn(II) or Cu(II), and moreover, the LIX84I-based PIM did not require the inclusion of an expensive plasticizer unlike its counterparts containing other commonly used base-polymers. The stability of these PIMs was also assessed over three transport cycles (i.e., simultaneous extraction and back-extraction), and it was observed that the transport rate decreased significantly when using D2EHPA-based PIMs, and in the case of LIX84I-based membranes it stabilized after the first cycle.

Introduction

Industrial separation of metal ions (e.g., zinc, copper, rare earth metals) is still currently performed mainly by solvent extraction, as it provides high selectivity and high enrichment factors [1]. However, this separation technique involves the use of large volumes of often toxic and volatile solvents, and moreover the extraction and back-extraction processes of the target species have to be performed in separate steps. Hence, greener, safer, inexpensive, and time-efficient alternative technologies are desirable.

Separation involving liquid membranes is an attractive alternative to conventional solvent extraction because it minimizes and even eliminates the use of solvents, reduces separation costs by drastically decreasing the amounts of extractants used and integrates the extraction and back-extraction processes into a single step by conducting these two processes simultaneously at the corresponding sides of the membrane [2]. The most frequently used liquid membranes are supported liquid membranes (SLMs) [3]. Polymer inclusion membranes (PIMs) are a type of liquid membranes which visually resemble SLMs and combine their advantages with improved long-term stability [4], [5]. These membranes have two main components, namely an organic liquid phase and a base-polymer. The membrane liquid phase usually contains an extractant (also referred to as carrier), responsible for forming a complex or ion-pair with the chemical species of interest, and sometimes a plasticizer or modifier is also added to the membrane composition to improve the performance of the membrane by increasing its flexibility or the solubility of the extracted target chemical species – carrier adduct in its liquid phase, respectively. The base-polymer holds the liquid phase between its entangled chains and provides mechanical strength to the membrane. The two most commonly used base-polymers in PIMs are cellulose triacetate (CTA) [6], [7], [8], [9], [10], [11], [12] and poly(vinyl chloride) (PVC) [13], [14], [15], [16] because they are commercially available and have good compatibility with a wide range of extractants [17]. More recently, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) has also been used to prepare PIMs successfully, and moreover it has been demonstrated that the PIMs containing this co-polymer performed better than most CTA- or PVC-based PIMs in terms of extraction rate and stability [18], [19]. This enhancement in performance could be attributed to the PVDF-HFP chains being more flexible, thus promoting better membrane permeability. However, PVDF-HFP is significantly more expensive than PVC and CTA, which can be viewed as a disadvantage in the scaling-up of PIM fabrication for industrial applications.

In this context, the aim of this work was to assess the feasibility of poly(styrene-block-butadiene-block-styrene) triblock co-polymer (SBS) as a base-polymer since it is significantly cheaper than PVDF-HFP and is readily available commercially. SBS (Fig. 1) is a thermoplastic elastomer with excellent mechanical strength (over 10 MPa tensile strength) and flexibility (over 1000% stretching ability) [20]. Moreover, it has good resistance to both acidic and basic media and is soluble in common organic solvents (e.g., tetrahydrofuran) [21]. All these features indicate that SBS is a promising candidate as a base-polymer of PIMs and its use could reduce the cost of industrial PIM-based separation.

The present study examined the compatibility of SBS with commonly used extractants and compared the extraction and transport properties of successfully prepared SBS-based PIMs with PIMs containing commonly used base-polymers. The SBS-based PIMs were also characterized by water contact angle measurements, atomic force microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The stability of the newly developed SBS-based PIMs was also studied.

Section snippets

Reagents and solutions

Polystyrene-block-polybutadiene-block-polystyrene triblock co-polymer with styrene (SBS, Lot#MKCF0088, Aldrich) was used as base-polymer, and the following chemicals were used as extractants: di(2-ethylhexyl)phosphoric acid (D2EHPA, 97%, Sigma Aldrich), LIX84I (BASF, ~49% 2-hydroxy-5-nonylacetophenone in a high flash point hydrocarbon diluent, Lot# 0016898649), Aliquat 336 (Aldrich, USA), tri-n-butylphosphate (TBP, 97%, Sigma Aldrich), tri-n-octylamine (TOA, 98%, Aldrich),

Study of different extractants and PIM optimization

SBS consists of a thermoplastic elastomer [24] and it has been reported that a film of SBS looks transparent despite exhibiting a microphase-separation [20]. However, to the best of our knowledge, the suitability of SBS as a base-polymer in PIMs has not been reported to date. Hence, with the aim of assessing the compatibility of SBS with a wide range of extractants commonly used in PIMs [5], different types of extractants were selected, namely, acidic and/or chelating extractants (i.e., LIX84I,

Conclusions

The triblock co-polymer SBS was used as a base-polymer in the preparation of PIMs for the first time. Commonly used extractants in PIMs were selected to assess their compatibility with SBS, and those with relatively long aliphatic chain(s) and low polarity due to the lack of strong acidic groups such as LIX84I and D2EHPA exhibited good compatibility with the non-polar SBS unlike more polar extractants such as the ionic liquids Aliquat 336, Cyphos® IL 101 and Cyphos® IL 104. PIMs containing

Acknowledgements

X. Xiong acknowledges the China Scholarship Council (CSC) for sponsoring his stay in Australia. The authors would like to thank the Australian Research Council (ARC) and Melbourne Water Corporation for funding this research (grant LP160100687).

Declaration of Competing Interest

The authors declare no competing interests.

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